Recording materials for heat-sensitive recording generally are of a leuco color-development type. With this type of heat-sensitive recording material, an image is formed on the surface of the material through a color reaction caused by heat-fusion of either a leuco dye or an acidic substance dispersed in the heat-sensitive recording layer. This type of recording materials are disadvantageous in that fixation of the recorded image is insufficient and this results in undesired color development at a non-image areas on rough handling or heating, or by other causes, and in degrading the recorded image.
In recent years, diazo color-development type recording materials have been investigated extensively for the purpose of overcoming these disadvantages. For example, recording materials are described in which a recording layer contains color-developing components comprising a diazo compound, a coupling component, and a basic substance (including substances that becomes basic on heating) in JP-A-57-123086 (The term "JP-A" as used herein means an "unexamined published Japanese patent application".), Gazou Denshi Gakkaishi (Journal of Image Electronics Society), 11, 290 (1982), and so forth. With these recording materials, an image is firstly formed thereon by heating, and subsequently the material is irradiated with light to decompose the unreacted diazo compound to stop color development (namely, to fix the image).
However, even with recording materials utilizing a diazo compound, pre-coupling (or color reaction) slowly proceeds during storage before recording. This frequently causes undesired color development (or fogging).
Various improvements have been made to avoid undesired color-development. For example, one of the color-developing components is formed into discontinuous particles (or a solid dispersion), or is incorporated in a separate layer (JP-A-57-123086). These improvements attempt to keep one component separated from the other component to inhibit the pre-coupling reaction. Although satisfactory storability (so-called raw storability) is achieved with these improvements, heat-responsiveness (fusibility on heating), which is an important property, tends to be deteriorated.
Techniques for simultaneously improving the properties of raw storability and heat-responsiveness are known in which one of the color-developing components is encapsulated to separate it from the other component using a non-polar wax material (JP-A-57-44141 corresponding to U.S. Pat. No. 4,400,456, and JP-A-57-142636), or a hydrophobic polymer (JP-A-57-192944).
A wax material or a polymer material is dissolved in a suitable solvent in the above encapsulation, and a color-developing component or another component is dissolved or dispersed therein to form capsules. Accordingly the capsules in these methods are different in principle from usual capsules which are composed of a core material and a shell covering the core material. In capsules which are formed by dissolving a color-developing component, the color-developing component does not necessarily become the core material of the capsule but is mixed uniformly with the encapsulating substance. As a result, pre-coupling slowly proceeds at the wall interface, resulting in the disadvantage of insufficient raw storability. On the other hand, in capsules which are formed by dispersing a color-developing component, a color reaction occurs when the capsule wall is melted by heat. This results in the disadvantage of insufficient heat responsiveness. Furthermore, another problem in production is involved because the solvent used for dissolving or dispersing the wax substance or the polymer substance must be removed after the capsules have been formed.
Methods to offset these disadvantages are disclosed in which at least one of the components involved in the color reaction is incorporated in a core material of a microcapsule, and a wall is formed around the core material by polymerization to form microcapsules (JP-A-59-190886 corresponding to U.S. Pat. No. 4,650,740 and JP-A-60-6493).
On the other hand, acylacetoanilide type compounds, which are used widely as a coupling component for azo pigments with yellow to orange colors, are not necessarily satisfactory in terms of the light-fastness of the obtained images.
Known methods for increasing the light-fastness of images include addition of a discoloration-preventing agent or a UV-absorbing agent, improving fixation of the image to substrates, insolubilization in solvents (or pigment formation), and so forth.
The introduction of Cl, OCH.sub.3, etc. into the aniline moiety of a coupling component is said to improve light-fastness. The compounds below are known therefor. Naphthol AS-LG (Color Index (CI) No. 37615): ##STR2##
These compounds, however, have insufficient oil solubility. This results in disadvantages because a high concentration emulsion thereof cannot be prepared easily with the compound dissolved in a solvent or a plasticizer, and because a uniform and stable emulsion cannot readily be obtained because of deposition of the compound during storage.
Furthermore, the compound below is also known. ##STR3## This compound, however, is slightly water-soluble because of its lower molecular weight although it is more soluble in a solvent. Therefore, a disadvantage occurs because this compound tends to undergo pre-coupling especially in the presence of a base.
Also, the following compounds are in the art. ##STR4##